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J C Sandstrom

Publications and source records attributed to J C Sandstrom.

5 recordsLinked to original sources
PubMed1991

Liver imaging at 1.5 tesla: pulse sequence optimization based on improved measurement of tissue relaxation times.

In order to predict the most sensitive MR imaging sequence for detecting liver metastases at 1.5 T, in vivo measurements of T1 and T2 relaxation times and proton density were obtained using multipoint techniques. Based on these measurements, two-dimensional contrast contour plots were constructed demonstrating signal intensity contrast between hepatic lesions and surrounding liver parenchyma for different pulse sequences and pulse timing parameters. The data predict that inversion recovery spin echo (IRSE) imaging should yield the greatest contrast between liver metastases and liver parenchyma at 1.5 T, followed by short tau inversion recovery (STIR) and spin-echo (SE) pulse sequences. T2-weighted SE images provided greater liver/lesion contrast than T1-weighted SE pulse sequences. Calculated T1, T2, and proton density values of the spleen were similar to those of hepatic metastatic lesions, indicating that the signal intensity of the spleen may be used as an internal standard to predict the signal intensity of hepatic metastases on T1- and T2-weighted images at 1.5 T.

Adult↗
PubMed1990

Separation of spin populations with gradient echoes as an aid in pulse sequence tuning.

Tuning of nuclear magnetic resonance pulse sequences with pulsed "crusher" gradients or phase cycling serves to remove unwanted spin populations from the data acquisition window. Verification that unwanted spin population are not detected is often determined by the absence of obvious artifacts in an image. This approach is unsatisfactory in some instances because signal contamination with unwanted spin populations may not be obvious. This is a particular concern with multiple-spin echo, volume-selective, and other multiple-pulse sequences. A solution to this problem is the separation of spin populations using gradient echoes, allowing the existence of unwanted populations to be easily observed separately. Tuning of a pulse sequence is straightforward when spin populations can be independently observed.

Humans↗
PubMed1989

A method for correctly setting the rf flip angle.

Currently the accepted method for setting the correct rf power levels to achieve 90 degrees and 180 degrees rf pulses for MR imaging is to peak the echo amplitude of a rf spin-echo sequence. The echo amplitude of this alpha-2 alpha pulse sequence is proportional to sin3 (alpha) and has a relatively broad maximum. Recently another method for setting the rf flip angle by maximizing the ratio of the stimulated echo to the primary echo amplitudes (in a 3 alpha sequence) demonstrated accuracy similar to that of the spin-echo method using a shorter repetition time. We present a new, more sensitive, and more accurate method for setting the correct rf power levels for 90 degrees and 180 degrees rf pulses. In this method, based upon the stimulated echo pulse sequence, we are able to accurately set the rf power to within +/- 0.1 dB by minimizing the signal amplitude of the third spin echo. This null method works for both selective and nonselective rf pulses of flip angle 90 degrees or 180 degrees, allowing the user to accurately adjust the relative amplitudes of the four rf pulse types within a single pulse sequence.

Calibration↗
PubMed1989

Projection flow imaging by bolus tracking using stimulated echoes.

Previous investigators have employed the concept of bolus tracking using either spin echoes or gradient echoes. In this paper we introduce two methods of bolus tracking using planar- and volume-selective stimulated echoes. The planar method employs a selective 90 degrees rf pulse which tags all spins in a particular plane. At a time tau 1 later, a nonselective 90 degrees rf pulse is employed, followed after a time tau 2, by another nonselective rf pulse. Only spins which experience all three rf pulses form a stimulated echo at time tau 1 after the third rf pulse. A balanced pair of flow-compensated dephasing (crusher) gradients further ensures that the stimulated echo is due only to the effect of all three rf pulses while minimizing flow dephasing. The first part of this gradient pair is applied after the initial rf pulse in the first tau 1 period to dephase the tagged spins. The second part of this gradient pair is applied after the third rf pulse to rephase the spins. Since the plane of the excited slice is orthogonal to the readout direction, flowing spins are imaged in an angiographic manner as they move away from the excited slice. A modification to this basic sequence excites only a small volume. In this manner, the suppression of stationary spins is effected by volume-selective excitation. In both the planar- and the volume-selective techniques, the excited spins undergo T1 and T2 relaxation during the tau 1 period but only T1 relaxation in the tau 2 period. In blood, where T1 is much greater than T2, keeping tau 1 as short as possible minimizes signal loss due to T2 dephasing. These methods demonstrate increased sensitivity compared to similar bolus tracking methods using either spin echoes or gradient echoes.

Blood Flow Velocity↗